Field Book: Pennsylvanian Plant Fossils of Illinois

Part 1

Field Book PENNSYLVANIAN PLANT FOSSILS OF ILLINOIS

Charles Collinson Romayne Skartvedt

Illinois State Geological Survey Educational Series 6

STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION

First edition 1960 Reprinted 1966

ILLINOIS STATE GEOLOGICAL SURVEY URBANA ILLINOIS John C. Frye, Chief

Printed by Authority of State of Illinois, Ch. 127, IRS, Par. 58.25.

Field Book PENNSYLVANIAN PLANT FOSSILS OF ILLINOIS

Charles Collinson Romayne Skartvedt

Illustrations by Marie E. Litterer

FOREWORD

This field book is intended to guide beginners in their collection and general classification of plant fossils. It illustrates the plant fossils most commonly found in Illinois and relates them to the plants of which they were a part. A list of publications that will furnish more detailed identification of specimens is included. The book has been prepared in response to numerous inquiries to the Illinois State Geological Survey from amateur collectors.

Information has been drawn from numerous sources. The works of Hirmer, Janssen, Lesquereux, Noé, and Langford have been particularly useful.

We are especially indebted to Dr. Robert M. Kosanke, paleobotanist at the Illinois State Geological Survey, and Dr. Wilson N. Stewart, professor of botany of the University of Illinois, for helpful suggestions and use of their libraries.

KEY TO PLANTS ILLUSTRATED ON TIME CHART

1. Foerstia. These fossils may be the earliest known occurrence of bryophytes, although some authors have referred them to the brown algae. After Dawson. 2. Psilophyton. A primitive vascular plant. After Dawson. 3. Lepidodendron. After Hirmer. 4. Sigillaria. After Hirmer. 5. Calamites. After Hirmer. 6. Sphenophyllum. After Fuller and Tippo. 7. Equisetum. The only living genus of scouring rushes. After Fuller and Tippo. 8. Megaphyton. An ancient true fern. After Hirmer. 9. Modern tropical tree fern. After Fuller and Tippo. 10. Medullosa. An ancient seed fern. After Stewart. 11. Williamsonia. An extinct cycad-like tree. After Sahni. 12. Cycas. A modern cycad. After Chamberlain. 13. Baiera. A fossil leaf genus of ginkgo, whose only living representative is the species Ginkgo biloba, saved from extinction by careful cultivation in China. Several specimens of this “living fossil” were presented to this country by the Chinese and are now flourishing on many college campuses, including that of the University of Illinois. After Mägdefrau. 14. Cordaites. After Grand Eury. 15. Lebachia. A “transition conifer,” forerunner of present day conifers. After Mägdefrau. 16. Pinus. Modern pine. After Mägdefrau. 17. Acer. Common maple, an angiosperm whose leaves are also found among Tertiary fossils. After Mägdefrau. 18. Rosa. The prairie rose, an angiosperm. 19. Campsis. Trumpet vine, an angiosperm.

Time Units Era and Years CENOZOIC “Recent Life” Pleistocene Pliocene Miocene Oligocene Eocene Paleocene MESOZOIC “Middle Life” Cretaceous 70 million herbaceous lycopods Jurassic 25 million yellow-green algae selaginellids Triassic 30 million cycad-like plants Equisetum PALEOZOIC “Ancient Life” Permian 25 million transition conifers pines, spruces, firs, etc. Equisetites Pennsylvanian 25 million ferns related to modern families bryophytes Mississippian 30 million seed ferns calamites cycads Devonian 55 million cordaites sphenophyllids ancient ferns Silurian 40 million psitopsids Foerstia Ordovician 80 million red algae green algae Cambrian 80 million PROTEROZOIC and ARCHAEOZOIC ERAS 4½ billion years blue-green algae chemosynthetic bacteria? NO CERTAIN FOSSILS KNOWN Plants Algae yellow-green algae brown algae red algae green algae blue-green algae Mosses bryophytes Foerstia Vascular Plants “Whisk Ferns” psitopsids Club-mosses scale and seal trees quillworts herbaceous lycopods Scouring Rushes, Horsetails Equisetites Equisetum calamites selaginellids Ferns and Seed Plants Ferns ancient ferns ferns related to modern families Gymnosperms Cycads seed ferns cycads cycad-like plants Ginkgo Conifers cordaites transition conifers pines, spruces, firs, etc. Flowering Plants

Field Book PENNSYLVANIAN PLANT FOSSILS OF ILLINOIS

Charles Collinson and Romayne Skartvedt

Plants that flourished 200 million years ago have made Illinois one of the best known fossil collecting sites in the world. The unusual abundance and preservation of these fossils in the northern part of the state have brought collectors to Illinois from many countries, and prized specimens from that area may be seen in science museums throughout the world.

The remarkable fossils represent plants that lived during the geologic period called the Pennsylvanian or Coal Age and are the result of special geologic conditions that occurred repeatedly during the period.

At the beginning of the Pennsylvanian Period, Illinois was part of a vast lowland that stretched for hundreds of miles to the north, south, and west, and was bordered on the east by highlands. At times much of the plain was swampy and, because the climate was relatively warm and moist, great jungles of fast growing trees, shrubs, and vines covered the landscape. As successive generations of plants lived and died, plant material fell into the swamp waters and, protected there from decay, accumulated.

Frequently during the period, seas spread over the swampy lowlands, submerging the forests and covering them with mud. Each submergence lasted only a short time, geologically speaking. When the seas withdrew, the deposits of sand and mud left behind were cut by streams that carried fresh sand and mud from the eastern highlands. The streams eventually became clogged with sediments and when the lowland was again depressed swamp conditions returned and forests grew afresh. Such a cycle of deposition was repeated again and again during Pennsylvanian time, and after burial each layer of plant material gradually lost most of its liquids and gases and was slowly converted into one of the numerous coal beds presently found in Illinois.

In some places in the state conditions existed that were especially favorable for preservation of plants, and there delicately preserved fossils are found in great numbers. In the most favorable areas, such as in northern Illinois, the plants are preserved in stony nodules called concretions, but they also may be found separately as molds, casts, or petrifactions.

Molds (concave surfaces) and casts (convex surfaces) are fossilization phenomena in which the actual plant, embedded in the surrounding background rock, was dissolved, leaving a hollow space (mold) that subsequently filled with other material. A cast was thus formed that preserved the plant’s external features.

Most petrifactions are fossils in which silica, carbonate, or other material permeated or replaced the internal structures of the plant and preserved them so well that in most specimens the finest cellular details can be observed. Compressions, another kind of petrifaction, are the pressed carbonized remains of the plant itself.

PENNSYLVANIAN FLORA

The far-reaching Pennsylvanian swamplands had abundant species of trees and other plants that long since have become extinct. Today’s common deciduous trees were not present; flowering plants had not yet evolved. Instead, the tangled forests were dominated by giant ancestors of presently existing club-mosses, horsetails, ferns, conifers, and cycads. The undergrowth also was well developed, consisting mainly of ferns, fernlike plants, Sphenophyllum, and small club-mosses. The plant fossils give no indication of seasonal variations. The forests, evidently always green, grew rapidly and abundantly, with foliage of unprecedented size and luxuriance. Land animals were just beginning to develop and included sluggish, salamander-like amphibians, large primitive insects, and a few small reptiles. The insects flourished as never before or since in the damp forests and attained remarkable size. Insects more than four inches long were common and some are known to have been more than a foot long with a wingspread proportionately broad. Ancestors of the modern spiders, scorpions, centipedes (one fossil found in Illinois was twelve inches long), cockroaches, and dragonflies are represented by several hundred species.

Part 2

The fossilized plants of Pennsylvanian time belonged to only a few main categories: scale and seal trees, ancient scouring rushes (horsetails), herbaceous Sphenophyllum, ferns, seed ferns, and cordaitean trees.

SCALE AND SEAL TREES (Plate 1)

Scale and seal trees were abundant during the Pennsylvanian Period and were important contributors to coal beds. Although distantly related to the diminutive club-mosses and ground pines of the present, the trees grew on straight, slender trunks to heights of more than a hundred feet.

Scale trees were so called because their numerous, closely set, spirally arranged leaves left scarred “cushions” on the branches and trunk, making them appear scaly. Seal trees derived their name from the signetlike appearance of their leaf cushions. The two best known types belong to the genera Lepidodendron (scale tree) and Sigillaria (seal tree), and fossils of both are common in Illinois.

Lepidodendron had long, slender, somewhat tapering trunks. Some of the trees reached heights of more than 100 feet and measured more than two feet in basal diameter. The trunk ended in a spreading crown formed by repeated dichotomous branching. The leaves were awl-shaped or linear, ranging from one to 30 inches long.

The leaf cushions of Lepidodendron are diamond-shaped, longer than broad, and arranged in spiral rows around the trunk and branches. A different name, Lepidophyllum, is used for fossils of the long, bladelike leaf when it is found detached.

Spores were borne in long cylindrical cones at the tips of the branches. Those cones referred, or assigned, to the genus Lepidostrobus bore both small spores (microspores) and large spores (megaspores) in the same cones. Those in which only a large single spore, a somewhat seedlike structure, was developed in a spore sac (sporangium) are referred to the genus Lepidocarpon.

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